Venous access catheters provide venous access to the central circulatory system. Venous access catheters include central venous catheters, midlines dialysis catheters, implantable ports and peripherally inserted central catheters, also known as PICC lines. The access line or port with attached catheter is used for the delivery of intravenous fluids, medications such as chemotherapy drugs and antibiotics, and blood products. Venous access catheters may also be used as access mechanisms for blood sampling and the administration of contrast agents during diagnostic Computer Tomography (CT) procedures.
One type of venous access catheters, a PICC line, provides venous access to the central circulatory system through a peripheral vein. Central venous catheters or CVCs also provide access to the venous system but are inserted through a larger vein, closer to the heart. Yet another type of venous access catheter, the dialysis catheter, provides either acute or chronic venous access to the central circulatory system for filtering of blood during a dialysis procedure. Implantable ports provide subcutaneous access to the venous system via needle inserted septum defining a reservoir in fluid communication with a catheter shaft. PICCs, CVC, ports and dialysis catheter shafts come in a variety of configurations. These include single lumen, dual lumen and other multi-lumen configurations. They come in various lengths to accommodate different anatomy and catheter insertion sites.
Venous access devices are designed to remain within the patient for a period of days to a year or even longer, and can be accessed in an inpatient, outpatient or home setting. One of the most common complications resulting from long term implantation of venous access devices is the buildup of thrombus on the indwelling portion of the catheter due to blood platelet adhesion on the catheter shaft surfaces. In addition, a fibrin sheath, or fibrin layer, may form along the vein entry site or may grow over the catheter tip forming what is known as fibrin tail.
Thrombus can appear anywhere on a catheter surface which is exposed to the bloodstream, but clot formation is often most extensive at the distal section of the catheter near the tip and that portion of the shaft proximate to the venotomy insertion site. Localized thrombus buildup on the catheter can result in various complications which compromise the performance of the indwelling device including increased catheter related blood stream infection, complete or partial catheter occlusion and non-laminar blood flow. Both adherent fibrin and platelet adhesion can interact to further promote thrombus build-up. Complications from fibrin and clot buildup may necessitate additional procedures to mechanically disrupt the clot mass or the administration of anti-thrombotic medications such as TPA or other thrombolytic drug. In some cases, the catheter must be removed and replaced.
The prevention of thrombus formation on catheter shaft surfaces has been the subject of much research and product development efforts in the medical device community. For example, attempts to prevent or minimize thrombus formation on catheter surfaces have focused on catheter coatings including anticoagulants, such as heparin The main disadvantage of coatings is the tendency of the coating to elude from the shaft after implantation resulting in reduced long-term protection against thrombus formation.
U.S. Pat. No. 6,127,507, which is incorporated herein by reference for all purposes, describes the use of certain fluoroalkyl surface-modifying macromolecules in admixture with elastomers for the manufacture of blood-contacting medical devices. Such macromolecules have shown to be clinically effective in the reduction in thrombosis formation. The admixtures, also known as surface modifying additives, when used are preferably synthesized in a manner that they contain a base polymer compatible segment and terminal hydrophobic fluorine components which are non-compatible with the base polymer. The compatible segment of the surface modifier is selected to provide an anchor for the surface modifier within the base polymer substrate upon admixture. While not being bound by theory, it is believed that the fluorine tails are responsible in part for carrying the surface modifier to the surface of the admixture, with the chemical resistant fluorine chains exposed out from the surface. The latter process is believed to be driven by the thermodynamic incompatibility of the fluorine tail with the polymer base substrate, as well as the tendency towards establishing a low surface energy at the mixture's surface. When the balance between anchoring and surface migration is achieved, the surface modifier remains stable at the surface of the polymer, while simultaneously altering surface properties.
Embodiments of SMM fluoropolymer additives used in the present invention may be synthesized using a multi-functional isocyanate, a multi-functional soft segment precursor reactive therewith, and a mono function polyfluoro-alcohol. The isocyanate is preferably, but not so limited to be di-functional in nature, in order to favour the formation of a linear SMM. Linear as opposed to branched or crosslinked SMM have better migration properties within the polyurethane substrate. A preferred diisocyanate for biomedical applications is 1,6-hexanediisocyanate. The soft segment precursor molecule is preferably di-functional in nature but not so limited to be di-functional, in order to favour the formation of a linear SMM. Again, linearity favours migration properties within the base polymer substrate. Examples of typical soft segment precursors include, polypropylene oxide polyols of molecular weight 1000, and polytetramethylene oxide diols of molecular weight 1000. SMM's are synthesized using a preliminary prepolymer method similar to the classical one used for polyurethanes. However, the subsequent step differs in that a chain extension is not carried out. A mono-functional oligomeric fluorinated alcohol is used to cap the prepolymer, rather than chain extend the prepolymer. The fluorinated alcohol preferably has a single fluoro-tail but is not limited to this feature. A general formula for the oligomeric fluoro-alcohol of use in the invention is H—(OCH2CH2)n—(CF2)m—CF3, wherein n can range from 1 to 10, but preferably ranges from 1 to 4, and m can range from 1 to 20 but preferably ranges from 2 to 12. A general guide for the selection of “n” relative to “m” is that “m” should be equal or greater to “2n” in order to minimize the likelihood of the (OCH2CH2)n segment displacing the (CF2)m—CF3 from the surface following exposure to water, since the former is more hydrophilic than the fluorotail and will compete with the fluorotail for the surface. Without being bound by theory, the presence of the (OCH2CH2)n segment is believed to be important to the function of the SMM because it provides a highly mobile spacer segment between the fluorotail and the substrate. This is important in order to effectively expose the fluorosurface to, for example, an aqueous medium. Examples of typical oligomeric fluoroalcohols include various fractions BA-L, BA-N, FSO-100 and FSN-100 (DuPont de Nemours, Wilmington, Del.).
In some embodiments, the catheter comprises a polymeric material comprising a fluoropolymer comprising terminal polyfluoro-oligomeric groups, wherein the fluoropolymer is characterized by a polystyrene equivalent weight average molecular weight (Mw) greater than 13,000 Daltons (13 kDa). In particular embodiments, the fluoropolymer can contain less than 10% (w/w) (e.g., from 0% to 1.5%, 0% to 2%, 0.1% to 2.2%, 0.3% to 3%, 0% and 5%, or 0.5% to 5% (w/w)) trimer formed by reaction of one diisocyanate with two perfluorinated alcohols to form a low molecular weight fluoropolymer component containing no soft segment. In certain embodiments, the fluoropolymer can have a polystyrene equivalent weight average molar mass, Mw, of from 2,000 to 26,000 g/mole (e.g., 6,000±4,000, 8,000±4,000, 10,000±4,000, 12,000±4,000, 18,000±4,000, 20,000±4,000, 22,000±4,000, or 24,000±2,000 g/mole). In some embodiments, the fluoropolymer can have a polystyrene equivalent number average molar mass, Mn, of from 2,000 to 18,000 g/mole (e.g., 6,000±4,000, 8,000±4,000, 10,000±4,000, 13,000±2,000, 14,000±2,000, 15,000±2,000, or 16,000±2,000 g/mole). The fluoropolymer can have a polydispersity index of between 1.0 and 2.0 (e.g., a polydispersity of 1.1 to 1.4, 1.3 to 1.6, 1.35 to 1.55, 1.5 to 1.7, or 1.6 to 1.9). For example, the fluoropolymer can have a polystyrene equivalent weight average molar mass, Mw, of from 2,000 to 14,000 g/mole (e.g., 6,000±4,000, 8,000±4,000, or 12,000±2,000 g/mole), and/or a polystyrene equivalent number average molar mass, Mn, of from 2,000 to 12,000 g/mole (e.g., 6,000±4,000, 8,000±4,000, or 10,000±2,000 g/mole), and comprises between 0% and 3% (w/w) (e.g., from 0% to 1.5%, 0% to 2%, 0.1% to 2%, 0.1% to 2.2%, 0.3% to 2.2%, or 0.5% to 2.5% (w/w)) trimer. Alternatively, the fluoropolymer can have a polystyrene equivalent weight average molar mass, Mw, of from 14,000 to 26,000 g/mole (e.g., 18,000±4,000, 20,000±4,000, or 22,000±4,000 g/mole), and/or a polystyrene equivalent number average molar mass, Mn, of from 10,000 to 16,000 g/mole (e.g., 12,000±2,000 or 14,000±2,000 g/mole), and comprises between 0% and 3% (w/w) (e.g., from 0% to 1.5%, 0% to 2%, 0.1% to 2%, 0.1% to 2.2%, 0.3% to 2.2%, or 0.5% to 2.5% (w/w)) trimer. Fluoropolymer of desired size distribution and composition can be prepared, for example, by reducing the amount of diisocyanate used to make the fluoropolymer and/or by fractionating (i.e., by column chromatograph, dialysis, or extraction) the fluoropolymer.
Unlike coating technologies which are prone to degradation over time due to elution, the admixture described in the '507 patent may be added to a base polymer to create a compound in which the admixture is uniformly dispersed throughout. When the compound is extruded to form a catheter tube, the admixture becomes integral to the catheter shaft i.e., evenly dispersed throughout the wall of the shaft including inner and outer wall surfaces and any cut surface. As such, the catheter is not subject to surface elution or coating wear, but instead is capable of maintaining thrombo-resistance levels throughout indwelling time of the device.
Early test results of venous access devices formed with the surface modifier admixture described in the '507 patent, confirmed a significant reduction in platelet and thrombus adhesion when compared to devices having no anti-thrombotic features. In a standard blood loop study using an AngioDynamics PICC catheter manufactured with the surface modifier admixture (trademark ENDEXO), there was an 87% reduction in platelet and thrombus adhesion when compared to a PICC catheter containing no anti-thrombotic additives or coatings, the ENDEXO PICC catheter showed only minimal thrombus. The control PICC catheter, on the other hand, showed extensive thrombus accumulation. It is believed that the fluorine macromolecules present in the admixture help to create a catheter surface to which blood platelets and fibrin growth do not easily attach.
In an effort to further optimize the thrombus reduction qualities of a catheter shaft formed from the polymer with the surface modifier admixture described above, the inventors believe that lowering the durometer of the base polymer material will further enhance the overall thrombo-resistance of the device. Durometer is a measurement of the hardness of the base polymer material. The Shore A scale is commonly used in softer polymer measurements, with a higher number indicating a harder material. A softer (lower) durometer base polymer exhibits a higher level of micro-hydration than a harder (higher) durometer material when exposed to body temperatures and fluids. In addition, the softer material will have a greater tendency to develop surface micro-pores or “micro-cracks” under in vivo conditions which enhance surface area presentation of the surface modifier. These pathways may enhance the presentation of the fluorine macromolecules toward the catheter shaft surfaces, further improving the overall effectiveness of catheter against thrombus. Thus, due to the preferential presentation of the fluorine macromolecules through the softer durometer polymer material of the shaft, the previously demonstrated thrombus reduction rates of up to 87% may be further improved.